There have been many prior attempts over many years to provide practical continuously variable transmissions.
One well known friction type, employs cooperating friction belts and pulleys of varying diameter while another involves cooperation between drive/driven shafts and bodies of continuously varying shape such as cones but the reliance on friction imposes significant limitations on torque while the friction belts or other components in frictional engagement can be subject to wear, requiring inconveniently frequent replacement, increasing the operating costs.
Another approach taught by WO9302302 published in 1993, teaches engagement between a conical pinion and a "hedgehog" wheel with spring loaded teeth which retract and emerge from recesses to accommodate pinion taper as the wheel is displaced along an inclined axis to maintain it in mesh with the conical pinion.
However, the prior proposal does not follow the laws of gearing. In consequence, it is believed that, in practice, a substantial amount of sliding friction will occur between the sprung teeth and the pinion teeth resulting in relatively rapid wear and energy losses.
Other prior approaches involving gears, such as epicyclic arrangements utilizing one-way clutches have resulted in a variable velocity output and relatively low reliability. Furthermore, engine braking is usually not possible.
Another, so-called frictionless approach, taught by U.S. Pat. Nos. 4,817,464 and 4,850,248 issued to Korban et al in 1989 has the disadvantage that the output is not of constant velocity.